Polymers are used extensively to make a variety of products which include blown and cast films, extruded sheets, injection molded articles, foams, blow molded articles, extruded pipe, monofilaments, fibers, and nonwoven fabrics. Many polymers that are used to form these products, such as polyolefins, are naturally hydrophobic or apolar and are chemically inert. For many uses, hydrophobicity is a disadvantage, particularly when printing with aqueous-based inks having a relatively higher surface tension than the surface energy of the polymeric substrate. For example, aqueous-based inks can have a surface tension of greater than or equal to above 45 dynes/cm, while the polymeric substrate can have a surface tension of about 30 dynes/cm. While substrate hydrophobicity may not be an issue with lower surface tension inks or solvent-based inks, still the apolar nature of the polymeric substrate will not promote good adhesion of these inks, either aqueous or solvent based, to the polymeric substrates, resulting in printed graphics that will easily rub off when exposed to shear.
Typically, the polymers used to form these products are poorly polar resulting in them being non-conducive to adhere most common ink compositions applied to the surface of the polymeric substrate. Also, these polymers are typically non-absorbent and unable to form a mechanically strong network with the ink composition after it is applied to the polymeric substrate.
Hydrophobic polymers, including polyolefins, such as polyethylene and polypropylene, can be used to manufacture polymeric fabrics which are employed in the construction of packaging articles and disposable absorbent articles such as diapers, feminine care products, incontinence products, training pants, wipes, and so forth. Such polymeric fabrics are often nonwoven fabrics prepared by, for example, processes such as melt blowing, carding, co-forming, spunbonding, and combinations thereof.
Absorbent articles, especially personal care absorbent articles, such as diapers, training pants, and swimming pants, typically include an outer cover made from a nonwoven polymeric fabric. The outer cover of diapers, training pants, and swimming pants, for example, are difficult to print on in a fast and economic manner that is amiable to efficient machine production. More particularly, it is difficult to get good ink adhesion to such hydrophobic polymeric substrates. In particular, it has been difficult to print colored graphics that are crockfast onto the polymeric substrates, especially through conventional printing methods such as a flexographic process. It has been even more difficult to print colored graphics that are crockfast onto the polymeric substrates via digital printing processes and digital inks.
With training pants, such as PULLUPS brand training pants manufactured by the assignee of record, Kimberly-Clark Corporation, it is desirable to make the product as aesthetically attractive and appealing to the consumer as possible to wear during training of the child to progress from diapers to underwear. One means to make this product more appealing is to print in bright colors a number of designs on the exterior cover of the training pant. However, in the past, it has been difficult to directly print colored inks onto the exterior surface of the training pant without costly processes, such as over-lacquers to protect the ink from abrasion. As a result, it has typically been necessary to print these colored designs on an underlying film layer and then superimpose the polymeric substrate, as an added layer over the top of the printed film layer such that the color designs can be viewed, albeit somewhat diffusely, through the polymeric substrate.
Accordingly, there is a need to improve adhesion of inks to outer covers on diapers, training pants, swimming pants, and other products that incorporate hydrophobic substrates. Also, a need exists for a process in which designs can be printed directly on the exterior surfaces of absorbent articles.
A need also exists to improve color vibrancy of the printed inks to outer covers on diapers, training pants, swimming pants, and other products that incorporate hydrophobic substrates. It follows that a need exists for a method to treat hydrophobic substrates so that ink usage is minimized while providing good color vibrancy and good ink rub resistance.